1. Field of the Invention
The present invention relates to high-analysis low pH liquid fertilizer compositions and the methods of preparing said compositions from phosphoric acid, urea, potash and other common agricultural materials.
Modern agriculture is based on appropriate fertilization. The cost of acquiring and applying nutrients is dominant, and much effort on the part of many investigators has been directed at improving the cost-effectiveness of fertilizer systems and practices. As a result of these efforts, liquid fertilizers are growing in acceptance because of economics in appication and placement; products of higher analysis have become available, leading to cost reduction all along the line from the point of manufacture to application in the field. The present invention utilizes a novel concept to carry these favorable factors much farther than previously possible.
2. Prior Art
Much work has been done in the past on the production of high analysis liquid fertilizers. For example, U.S. Pat. Nos. 2,770,538 and 2,869,996 describe liquid products made from phosphoric acid neutralized with ammonia and potassium hydroxide, to which is added urea or ammonia plus nitric acid to furnish additional nitrogen. U.S. Pat. No. 2,814,556 shows how to combine urea, ammonia, phosphoric acid and potassium chloride to make neutral liquids of high analysis containing ammonium phosphate. U.S. Pat. No. 3,022,153 teaches the combination of urea, wet-process phosphoric acid and ammonia; stabilization with amines is required if storage periods greater than a few days are contemplated. Complexes of urea and micro-nutrient metals are described in U.S. Pat. No. 3,640,698; to these may be added moderate amounts of other nutrients to form liquid systems of low pH and low analysis. In U.S. Pat. No. 3,713,802 a process is described for reacting urea and wet process phosphoric acid, separating the precipitated urea-phosphate and ammoniating it to convert it into commercially useful grades of liquid or solid fertilizer; in this process, separation of the urea-phosphate by precipitation and filtration is required to remove contaminants present in the phosphoric acid. Similarly, U.S. Pat. No. 3,723,086 discloses reacting merchant grade wet process phosphoric acid and urea followed by ammoniation to produce liquid fertilizers. U.S. Pat. No. 3,918,952 uses potassium chloride, urea and ammonium polyphosphate to make high analysis liquids; in these products, potassium chloride is the only form of potash compatible with the other ingredients, and relatively expensive polyphosphates must be used.
Generally in the prior art, high analysis liquids are achieved by using expensive furnace grade phosphoric acid, or by removing contaminants from the cheaper wet process phosphoric acid by costly processing, or by converting orthophosphoric acid to polyphosphoric acid by heat treatment. The products are usually neutralized to pH values ranging from 6 to 8.7 for stability and achievement of the required analysis, with some exceptions as noted above (U.S. Pat. No. 3,640,698) in which only low concentrations of total primary nutrients (N+P+K below 25%) can be realized. The field of the present invention is low pH fertilizers of high total nutrient content (25% or more, reaching values of 50% or more). These products are clear liquids, or suspensions when K exceeds the limit of solubility, made from conventional commercial materials.
In the present invention, urea is used as the source of nitrogen and phosphoric acid as the source of phosphorous. The reaction of urea and phosphoric acid to produce urea phosphate is known. As recognized in the art, this reaction is the equimolecular reaction of urea and phosphoric acid to produce a coordination-type complex by the equation: EQU CO(NH.sub.2).sub.2 +H.sub.3 PO.sub.4 =CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4
Dilute aqueous solutions of urea phosphate have been described; see, for example, U.S. Pat. No. 4,013,446. The limit of solubility of this material in water at 25.degree. C. is 90 parts per 100 parts of water by weight, giving a solution containing 7% nitrogen and 17% phosphorous pentoxide; at 0.degree. C., the solubility and assay are appreciably lower. Solutions of higher concentration crystallize; this fact is used in British patent specification No. 1,149,924 to make solid urea phosphate. In the present work, stable liquid fertilizers are produced that are far more concentrated than the former references, without the crystallization of solid urea phosphate described by the latter.
Urea is also reacted with sulfuric acid to make certain fertilizer grades as shown in U.S. Pat. No. 4,116,664. In these cases, urea reacts with sulfuric acid to form urea sulfate, a known compound and liquified urea.
Liquid fertilizers provide opportunities for further savings in several key areas including:
1. Production of higher analysis compounds PA1 2. Use of lowest cost components PA1 3. Provision of formulating flexibility so that the desired precise nutrient ratios can be made from a minimum number of simple components PA1 4. Simplification of the compounding process and equipment so that it can be moved as close as possible to the point of application PA1 5. Incorporation of trace and secondary elements from low-cost inorganic sources PA1 6. Maximizing agronomic efficiency, especially with regard to retention and utilization of nitrogen in the soil and phosphate availability. PA1 (1) admixing: PA1 (2) reacting said urea and said acid exothermally and without external heating, and PA1 (3) recovering a liquid product having a total of N, P.sub.2 O.sub.5 and K.sub.2 O analysis of at least 25 weight % of said product. PA1 1. The system is neutralized, usually with ammonia, so that urea phosphate does not exist, the urea being displaced by ammonia. PA1 2. Urea is decomposed to form ammonia, which serves to neutralize the phosphoric acid. PA1 3. Urea phosphate is formed, crystallized and separated from the reaction mixture. PA1 4. Urea phosphate is formed in dilute enough solution so that it remains dissolved.
All of these elements have been addressed in the current invention, and what results is a novel method of producing novel liquid fertilizers.
It is an advantage of the present invention that liquid fertilizers having high total nutrient content according to commercial standards may be prepared. It is a further advantage of the invention that these liquid fertilizers are prepared autothermally, without recourse an external heating. The term"autothermally" is used herein to indicate an exothermic reaction. It is a feature of the present invention that the liquid fertilizers may be prepared from a limited number of components. It is a particular feature that the liquid fertilizers may be manufactured in simple equipment that is readily available and low in cost, using straight-forward precedures that can be easily mastered by compounders. It is a further feature of the present invention that the liquid fertilizers may be made quickly. It is a particular advantage of the present process that commercial grades of components may be satisfactorily employed to produce the fertilizers.
It is a further advantage that the liquid fertilizers of this invention exhibit commercially acceptable stability over the full range of nutrient content. In addition, it is a particular advantage that the present invention allows inclusion of secondary and trace elements in stable and available form without prior-art chelated or complexed products.
One of the features of the present invention is the preparation of a complete range of fluid fertilizers containing combinations of the primary nutrients, nitrogen, phosphorus and potassium, wherein N may range up to 30 weight percent, P.sub.2 O.sub.5 up to 50 weight percent and K.sub.2 O up to 30 weight percent.
A further most important feature of the present invention is to provide fertilizers of improved agronomic efficiency, in which nitrogen and phosphate availability are maximized.
The first feature of the products of this invention that contributes to improved agronomic efficiency is the lowering of soil pH locally, which substantially improves the availability and mobility of metallic trace elements. This effect is well known; it has been used in the past to increase trace element availability, but not in the form of this invention, employing acid fertilizers.
A second factor of unique importance in the enhancement of agronomic efficiency is the protective effect of acid on the mobility of phosphate ions in the soil. In many instances, calcium ions are present in the soil in a form and to a degree that lead to the formation of insoluble calcium phosphate. This is substantially phosphate rock, similar to that from which the phosphate was originally freed by the action of sulfuric acid. Acid in the formulations of this invention will achieve the same effect, rendering the phosphate ions free and available.
A third element in the improvement of agronomic efficiency relates to the retention of nitrogen in the soil. The loss of nitrogen as ammonia to the atmosphere is a well-known phenomenon in agriculture, and one of the advantages of urea over ammonia is that this loss is greatly reduced. However, urea is converted to ammonia by decomposition in the soil in an essential first step toward ultimate utilization by plants; in this process, ammonia will still be lost to the atmosphere. The acid fertilizers of this invention delay and reduce this loss by at least two mechanisms. First, the hydrolysis of urea and conversion to ammonia is slowed by the fact that it is present as an acid reaction product. Beyond this, the acid environment created by the use of the products of this invention provides sites to which ammonia is strongly attached as soon as it is formed; as a result, the loss of ammonia into the atmosphere is substantially reduced. This is a factor of considerable agronomic significance.